Comparative life-cycle energy payback analysis of multi-junction a-SiGe and nanocrystalline/a-Si modules

Despite the publicity of nanotechnologies in high tech industries including the photovoltaic sector, their life-cycle energy use and related environmental impacts are understood only to a limited degree as their production is mostly immature. We investigated the life-cycle energy implications of amorphous silicon (a-Si) PV designs using a nanocrystalline silicon (nc-Si) bottom layer in the context of a comparative, prospective life-cycle analysis framework. Three R&D options using nc-Si bottom layer were evaluated and compared to the current triple-junction a-Si design, i.e., a-Si/a-SiGe/a-SiGe. The life-cycle energy demand to deposit nc-Si was estimated from parametric analyses of film thickness, deposition rate, precursor gas usage, and power for generating gas plasma. We found that extended deposition time and increased gas usages associated to the relatively high thickness of nc-Si lead to a larger primary energy demand for the nc-Si bottom layer designs, than the current triple-junction a-Si. Assuming an 8% conversion efficiency, the energy payback time of those R& D designs will be 0.7-0.9 years, close to that of currently commercial triple-junction a-Si design, 0.8 years. Future scenario analyses show that if nc-Si film is deposited at a higher rate (i.e., 2-3 nm/s), and at the same time the conversion efficiency reaches 10%, the energy-payback time could drop by 30%. Copyright (C) 2010 John Wiley & Sons, Ltd.